This invention relates to piezoelectric transducers for ink jet heads and, more particularly, to lead-zirconium-titanate (PZT) thin-film transducers having a perovskite crystal structure.
In the Hoisington et al. U.S. Pat. No. 5,265,315, the disclosure of which is incorporated herein by reference, the preparation of thin-film piezoelectric transducers for ink jet heads used in ink jet systems is described. In the preparation of such transducers, one or more electrodes are formed on a substrate and a thin film of PZT piezoelectric material is applied to the electroded substrate by a sol gel process of the type described, for example, in the publication entitled "Preparation of Pb(Zr,Ti)O.sub.3 Thin Films by Sol Gel Processing: Electrical, Optical, and Electro-Optic Properties" by Yi, Wu and Sayer in the Journal of Applied Physics, Vol. 64, No. 5, Sept. 1, 1988, pp. 2717-2724. As described in the Hoisington et al. patent, the thickness of a PZT thin-film transducer should be the minimum necessary to withstand stresses applied to the film during ink jet operation and, for ink jet systems having orifice and ink chamber sizes in the general range described hereinafter using inks having operating viscosities in the range of about 1-40 cps, the PZT film should have a thickness in the range of about 1-25 microns. If the film thickness is greater than a few microns, it is preferably deposited in several layers to avoid cracking and to assure a small perovskite grain size.
It has been found, however, that the piezoelectric performance of PZT films deposited in this manner can be degraded by the tendency of the preferred perovskite form of PZT to nucleate in a nonuniform manner at the film surfaces or to be pre-empted by nucleation and growth of a nonpiezoelectric "pyrochlore" phase. Consequently, with patterned electrodes applied to the surfaces of the PZT film, there are variations in the piezoelectric properties in the regions adjacent to the electrodes, producing performance variations. Furthermore, the processing temperature required to initiate nucleation in the region adjacent to an electrode is highly dependent upon the choice of the electrode material and also tends to be higher than a temperature which is desirable to minimize loss and migration of lead from the PZT film.